From Open Door Networks:
The DoorStop X Firewall protects your Mac from undesired access from the Internet, logging both allowed and denied access attempts. Its easy-to-use interface means less chance of mistakes, and its wide range of addressing and service options gives you fine-grained control over how services are protected. DoorStop enhances the firewall built into Mac OS X and works with Mac OS X Server as well. Available standalone, or as part of the comprehensive DoorStop X Security Suite.